Nato has formally invoked its mutual defence clause after the United States produced "conclusive" evidence that
Saudi-born militant Osama Bin Laden was behind the 11 September attacks on New York and Washington.

This is an unprecedented moment in Nato's history - a reaffirmation of the alliance's founding principle that an attack upon one is an attack upon all.

It is clear that all roads lead to al-Qaeda and pinpoint Osama Bin Laden as being involved in [the attacks]

George Robertson

Nato Secretary-General George Robertson says the alliance's role will be to anchor the international coalition that is being built to support American military action.

Last month, Nato had already made it clear that it would invoke Article Five of its basic treaty if there was proof that the attacks in New York and Washington were carried out by some foreign threat.

Practical help

The Bush administration has set out at least some of its evidence to its Nato allies - and it is clear that they have collectively been convinced that this evidence points in the direction of Osama Bin Laden's shadowy al-Qaeda network.

Diplomatic support is important, but the question remains what practical assistance Nato countries intend to give to the United States as it prepares its military riposte.

In purely legalistic terms, Nato members are obliged to give whatever assistance the United States requires.

In practice there will be no Nato military response as such.

Some countries have already been helping to facilitate the US military build-up around Afghanistan.

A very few Nato countries may be asked for highly specialised practical help.

It is clear for example that Britain is likely to be involved in any US operation, possibly contributing special forces units.

But given the precisely targeted nature of the US military response and the extraordinary secrecy of this type of warfare, it is not soliciting widespread military assistance from its Nato allies.